A touch panel device detects a touch of an instructing body such as a finger and specifies the coordinates of the touched point, and it is considered as excellent user interface unit which can be attached to display devices so as to substitute for mechanical keyboards and mouse of an information processing apparatus, and various coordinate detection types such as resistive film types and electrostatic capacitance types have been manufactured.
In coordinate detection types, a projected capacitive touchscreen type (hereinafter, referred to as PCT type) capable of detecting a touch of an instructing body even in a case where the front surface side of a touch panel including a touch sensor is covered by a protective plate, such as a glass plate, with several mm thickness. Since it is possible to dispose the protective plate on the front surface, the PCT type is excellent in toughness. Also, since there is no operating unit, the PCT type has advantages in long life. Since these advantages have been recognized, PCT type touch panels are attached on image display devices so as to be used as various touch input units for mobile communication devices, ATMs of financial institutions, car navigation devices, and so on (Japanese Patent Application Laid-Open No. 2008-134836).
Meanwhile, in the projected capacitive type, since a capacitance value (measurement values) is depending on variations in temperature, environmental changes (including rain or snow which falls in the open air), and so on, it is difficult to use an absolute threshold value to detect a touch on a touch sensor panel. For this reason, there is a method in which measurement values are measured as reference values (baseline values) in a state where there is no touch of a finger or the like, and a touch is detected based on amounts of changes from the baseline values (differences between measurement values and the baseline values), and there is a method in which if it is determined that there is no touch, baseline values are automatically updated so as to keep up with variations in surrounding environment such as temperature (for example, Japanese Patent Application Laid-Open No. 2007-208682). If the baseline values have deviations (if the difference between the baseline value and the measurement value obtained when there was no touch are large), it becomes impossible to correctly detect a touch. For this reason, it is important to correct the deviations of the baseline values. There is a method in which, in order to automatically correct baseline values in a case where the baseline values have large deviations due to a sudden change in environment or the like, if the baseline values are considered as being abnormal, the baseline values are updated (for example, Japanese Patent Application Laid-Open No. 2012-150747).